An image forming apparatus such as a copy machine, or a facsimile, an image reading apparatus such as a scanner, a color erasing apparatus which erases a printed sheet using a special toner, or the like frequently uses a sheet feed apparatus which continuously feeds a plurality of sheets into the apparatus.
This type of sheet feed apparatus loads a plurality of sheets in a loading unit in piles, and sends sheets one by one into the apparatus from the top of a bundle of sheets. Accordingly, the plurality of sheets which is loaded in the loading unit has to be loaded in a state of being separated one by one, respectively.
However, due to an operational error, or a mistake by a user, there may be a case where a bound sheet bundle of a plurality of sheets which is bound by a staple, a clip, glue, or the like, is placed in the loading unit. When the bound sheet bundle is carried into a main body of the apparatus such as the image reading apparatus, or the color erasing apparatus, there is a problem in that a paper jam occurs in the apparatus, or an internal part of the apparatus is damaged. Therefore, in the sheet feed apparatus, a variety of measures for preventing the bound sheet bundle from being sent into the main body of the apparatus is taken.
For example, a “lift” of a sheet bundle, which occurs when a bound sheet bundle is fed, is detected by a “lift” detection unit, using a light emitting element and a light receiving element, as disclosed in JP-A 2009-292575.
However, in the sheet feed apparatus, paper dust is easily generated since sheets rub against each other. For this reason, when configuring the “lift” detection unit using the light emitting element and the light receiving element, there is a possibility that the generated paper dust is attached to the surface of the light emitting element or the light receiving element, and deteriorates a detection performance thereof.
In addition, the light emitting element and the light receiving element are arranged in positions in an upper part of a sheet feed tray in which the sheets are loaded, accordingly, the light emitting element and the light receiving element are usually exposed outside of the apparatus. For this reason, the light receiving element is easily affected by external light such as the westering sun, or room light, accordingly there is a possibility of an occurrence of a detection error due to the external light.
Therefore, a sheet feed apparatus is desired which includes a highly reliable “lift” detection unit which is not affected by paper dust, external light, or the like.